1. Field of the Invention
This invention relates to a pole change electric motor (PAM), and more particularly to a controlling system for changing the number of poles of a pole change electric motor to vary the rotational frequency of the motor.
2. Description of the Prior Art
Generally, the number (n) of an electric motor is represented by a following expression ##EQU1## where f is the frequency of a power source, and p is the number of poles. As can be seen from the expression, the rotational frequency (n) of an electric motor can be varied by changing the number of its poles. A pole change electric motor to which the present invention is directed makes positive use of this feature.
Electric motors of this type are used for applications wherein a load thereto varies: for example, a boiler forcing fan connected to an electric motor may run under a full load in the daytime and under a low load at night, and in such a case, from a point of view of saving power, the electric motor may be run, at night, in a lower rotational frequency (with an increased number of poles) in accordance with a low load, and in the daytime, it may be run in an increased rotational frequency (with a reduced number of poles) in accordance with a heavy load.
Now, a principle of such a pole change electric motor (hereinafter referred to as "PAM") will be described in more detail with reference to FIGS. 1A and 1B and FIG. 2.
FIGS. 1A and 1B illustrate relations between stator windings of a PAM and poles of a rotating magnetic field, and in those figures, reference symbols 1a, 1b, 2a, 2b, 3a, 3b, 4a, and 4b designate each a stator winding (modelled for one phase), and reference symbol 5 designates a pole (indicated by N or S) of a rotating magnetic field.
FIG. 2 illustrates a conventional pole change system for a PAM, and in this figure, reference symbol 6 designates stator windings of the PAM including windings 61a, 61b, 62a, 62b, 63a and 63b and having terminals U.sub.1, U.sub.2, V.sub.1, W.sub.2, W.sub.1 and V.sub.2, respectively. Reference symbols 7, 8 and 9 denote each a switch, V.sub.R, V.sub.S and V.sub.T denote power supply voltages of R, S and T phases of a three-phase power source, respectively, and reference symbol O designates a neutral point of the three-phase voltages.
It is to be noted that FIG. 1A is a diagrammatic representation, in a modelled form, of a pole change electric motor having four poles therein while the motor is used as an electric motor having six poles therein by reversing the polarity of electric currents flowing through the coils 2b, 3a, 3b and 4a which are shown in broken lines in FIG. 1B. If description is given by way of an example of an electric current of the R phase, the coil 61b of FIG. 2 is connected between the terminal U.sub.2 and the neutral point and the direction of the electric current flowing therethrough does not change after changing of the number of poles. Accordingly, the coil 61b corresponds to the coil 1a, 1b, 2a or 4b of FIGS. 1A and 1B. On the other hand, the coil 61a is connected between the terminals U.sub.1 and U.sub.2 and the direction of the electric current flowing therethrough changes after changing of the number of poles. Accordingly, the coil 61 a corresponds to the coil 2a, 3a, 3b or 4a of FIGS. 1A and 1B. In this way, a PAM wherein the number of poles can be changed can be obtained by changing connections of some of its stator windings to change coil currents. While FIGS. 1A and 1B illustrate an example wherein the polarity of electric currents is changed, the polarity may otherwise be changed by changing phase currents or the polarity of the same.
Referring again to FIG. 2, the PAM motor is run at a low speed with the switch 7 closed and with the switches 8 and 9 opened, and on the contrary, it is run at a high speed with the switch 7 open and with the switches 8 and 9 closed to change the electric currents flowing through the stator windings 6 thereby to change the number of poles of the motor. However, with the conventional circuitry construction, if the switches 7, 8 and 9 are operated to change the number of poles in order to change over the motor from a low to a high speed running, the rotational frequency of the PAM motor will increase suddenly from a low speed to a high speed; such a sudden increase may be undesirable for some types of loads. For example, where a load to the motor is a forcing fan for a boiler, because a controlling speed for reducing an air flow by a vane or damper (hereinafter referred to only as a vane) mounted on the fan is slow, it cannot follow an increasing speed of an air flow due to such an increase of the rotational frequency of the motor, and as a result, the amount of air forced into the boiler is increased suddenly, resulting in sudden rise of the internal pressure of the boiler which may lead to explosion or misfiring of the boiler.